1. Field of the Invention
The present invention relates to surface acoustic wave elements, and more specifically, it relates to a surface acoustic wave element to be bonded to a package with metallic bumps and a manufacturing method thereof.
2. Description of the Related Art
Along with increases in the frequency of mobile radio communications, a surface acoustic wave apparatus used for the mobile radio communications is also required to be usable in a high-frequency range. The surface acoustic wave apparatus includes a surface acoustic wave element having a piezoelectric substrate and a package for accommodating the surface acoustic wave element. Because an acoustic velocity of the surface acoustic wave element on the surface of the piezoelectric substrate is several thousands m/second, the wavelength of an interdigital electrode of the surface acoustic wave element is as small as several xcexcm, when forming the surface acoustic wave element operating at approximately 800 MHz, for example. Therefore, there has been a problem that the absolute value of the electrode film thickness for optimizing characteristics of the surface acoustic wave element is reduced so that a loss due to an electrode resistance, i.e., an ohmic loss, increases.
In order to solve such a problem, a surface acoustic wave apparatus as shown in FIGS. 5A and 5B is disclosed in Japanese Unexamined Patent Application Publication No. 7-212175. In addition, FIG. 5B is a schematic sectional view along the lines of FIG. 5A connecting the dash-dot lines Pxe2x80x94P, Qxe2x80x94Q, and Rxe2x80x94R, and the dash-dot lines S and T of FIG. 5B indicate boundaries of the connection. In addition, in the drawings attached to this specification, in the same way as FIGS. 5A and 5B, the sections shown along the lines Pxe2x80x94P, Qxe2x80x94Q, and Rxe2x80x94R in the plan view are shown in the sectional view corresponding to the plan view by connecting them along the dash-dot lines S and T.
In a surface acoustic wave apparatus 201 in the prior art, on a piezoelectric substrate 202, an interdigital electrode 203 and reflector electrodes 204 and 205 disposed on both sides of the interdigital electrode 203 are arranged. Also, relay electrodes 206 and 207 are arranged to be electrically connected to the interdigital electrode 203. Furthermore, electrode pads 208 and 209 are arranged so as to be electrically connected to the relay electrodes 206 and 207, respectively. The electrode pads 208 and 209 correspond to parts to be electrically connected to electrodes of a package, and metallic bumps are arranged on the electrode pads 208 and 209. The surface acoustic wave apparatus 201 includes the interdigital electrode 203 and the reflector electrodes 204 and 205 among various electrodes mentioned above and conductive films 212 shown in FIG. 5B. As shown by the electrode pads 208 and 209 in FIG. 5B as representatives, on at least portions of the electrode pads 208 and 209 or the relay electrodes 206 and 207, second conductive films 213 are deposited. That is, in comparison with the interdigital electrode 203, portions of the relay electrodes 206 and 207 and the electrode pads 208 and 209 have an increased thickness, so that an ohmic loss due to an electrode resistance is reduced, thereby improving electrical characteristics.
By the way, because electronic components have recently been required to have a reduced size and height, a surface acoustic wave apparatus using a flip chip bonding technique is put into practical use. In the surface acoustic wave apparatus using the flip chip bonding technique, an electrode forming portion of the surface acoustic wave element opposes a mounting surface of a package, and electrodes of the surface acoustic wave element and electrodes of the package are bonded with metallic bumps. In this case, in order to increase the bonding strength between the metallic bump and the electrode pad, metallic films such as AU are disposed on the electrode pads of the surface acoustic wave element. On the metallic film made of AU, a metallic bump made of AU is disposed, which in turn is bonded to the package electrode surface. Alternatively, on the electrode pad, a metallic film made of Ag and having excellent solder wettability is disposed, so that a method in which solder bumps that are formed in the package in advance are bonded to the metallic films having excellent solder wettability is used.
When using the method described above, bonding wire is not required. Therefore, since the surface acoustic wave element is not required to have wire pads to be connected to the bonding wire, the planar area and height of the surface acoustic wave apparatus can be reduced.
Also, in the surface acoustic wave apparatus obtained by using the flip chip bonding technique, when a surface acoustic wave is actually vibrated, while the film thickness of electrodes other than electrodes to be propagated is increased, the ohmic loss is reduced, and losses and the reduction in Q of a resonator can be suppressed. In this case, first, the electrodes, in which a surface acoustic wave is vibrated and propagated, i.e., the interdigital electrode, reflector electrodes, bus bar electrodes, relay electrodes, and electrode pads are formed of the same conductive film. Next, in electrodes disposed in portions other than where a surface acoustic wave is vibrated and propagated, the second conductive film is deposited or the first conductive film is increased in thickness.
Therefore, when performing the bonding with the metallic bumps, after depositing the metallic films such as Au on the thick conductive films on the electrode pads, the metallic bumps such as Au may be formed thereon. Also, when performing the bonding with the solder bumps, on the plurality of deposited conductive films or on the electrode pads thickly formed by the increase in the conductive film thickness, the metallic films made of Ag and having excellent solder wettability are provided.
FIGS. 6A and 6B are a plan view and schematic sectional view for illustrating a conventional method using the flip chip bonding technique for manufacturing a surface acoustic wave apparatus operating at low frequencies, respectively.
According to the method, by patterning a conductive film 222, an IDT electrode 223, bus bar electrodes 224 and 225, reflector electrodes 226 and 227, relay electrodes 228 and 229, and electrode pads 230 and 231 are formed. Next, as shown in FIGS. 7A and 7B, on each of the electrode pads 230 and 231, a conductive film 232 and a metallic film 233 are deposited. The conductive film 232 is arranged for improving adhesion to the conductive film 222, and the metallic film 233 is arranged for improving the bonding strength between a metallic bump 234 shown in FIGS. 8A and 8B and the electrode pad.
Therefore, according to the method, after patterning the conductive film 222, the conductive film 232 and metallic film 233 have to be deposited.
On the other hand, when obtaining the surface acoustic wave apparatus operating at a high frequency range, as described above, in order to reduce the ohmic loss, on the conductive film 232, a conductive film 241 made of the same material as that of the conductive film 222 is deposited, and on the conductive film 241, the conductive film 232 and metallic film 233 have to be deposited, as shown in FIGS. 9A and 9B.
When manufacturing the surface acoustic wave apparatus operating at a low frequency range by bonding between the surface acoustic wave element and the package with the solder bumps disposed on the package instead of the metallic bumps made of Au, after the patterned conductive film 222 is formed on the piezoelectric substrate 221 as shown in FIGS. 6A and 6B, the metallic film 232 for improving the bonding strength to the conductive film 222 and a metallic film 242 to be a solder barrier are deposited on the electrode pads, and finally, a metallic film 243 having excellent solder wettability has to be deposited, as shown in FIGS. 10A and 10B. That is, a deposited structure made of three metallic layers has to be formed. Also, in this case, in order to reduce the ohmic loss of the surface acoustic wave apparatus operating at high frequencies, as shown in FIGS. 11A and 11B, on the metallic film 232, a conductive film 244 made of the same electrode material as that of the conductive film 222 is further deposited, and then, a multi-layered structure including a plurality of metallic films 242 and 243 has to be formed.
On the other hand, published application WO99/05788 discloses a surface acoustic wave apparatus, in which at least one of a bus bar and electrode pad includes a combination of a first conductive layer having Al as a principal ingredient, an intermediate layer, and a second conductive layer having Al as a principal ingredient. Therefore, when the electrode pad having the above-mentioned deposited structure with a large thickness, the mechanical strength is increased. In the WO99/05788 Publication, there is a description of a structure in that on the electrode pad, wire bonding or an Au bump is disposed.
As described above, when manufacturing a surface acoustic wave apparatus operating at a high frequency range, in any cases of the bonding between the surface acoustic wave element and the package with the metallic bumps on the surface acoustic wave element and of the bonding between the surface acoustic wave element and the package with the solder bumps disposed on the package, it has been necessary that one or more of the conductive films 241 and 244 are additionally deposited on a region other than where a surface acoustic wave is vibrated and propagated in comparison with the case of manufacturing the surface acoustic wave apparatus operating at a low frequency range. Therefore, there has been a problem that the manufacturing process is complicated and not cost effective.
In the structure disclosed in the WO99/05788 Publication, the electrode pad includes a combination of a first conductive layer having Al as a principal ingredient, an intermediate layer, and a second conductive layer having Al as a principal ingredient. However, in this structure, when forming the solder bump, since the top layer of the electrode pad is formed of the second conductive layer having Al as a principal ingredient, there has been a problem that the solder diffuses so that sufficient bonding strength cannot be obtained.
In order to overcome the problems describe above, preferred embodiments of the present invention provide a surface acoustic wave element and a manufacturing method thereof, the surface acoustic wave element being accommodated in a package by a flip chip bonding technique for use at a high frequency range, and is capable of simplifying the manufacturing process and reliably obtaining excellent electrical characteristics due to the reduction in an ohmic loss, and which is excellent in the bump bonding strength when using solder bumps.
In accordance with a first preferred embodiment of the present invention, a surface acoustic wave element is accommodated within a package and is to be bonded to the package with solder bumps provided on the package, the surface acoustic wave element includes a piezoelectric substrate, at least one interdigital electrode provided on the piezoelectric substrate, a pair of bus bar electrodes connected to the interdigital electrode, relay electrodes connected to the bus bar electrodes, electrode pads connected to the relay electrodes, and electrically connected to the package, first metallic films provided on the electrode pads for improving bonding strength to the solder bumps, and second metallic films made of the same material as that of the first metallic film, and provided on at least one of the bus bar electrodes and the relay electrodes.
Preferably, the first and second metallic films have a multi-layered structure formed by depositing a plurality of metallic layers. In this case, the top metallic layer of the first and second metallic films is made of a metallic material having excellent bonding properties to solder bumps while the other metallic layers are made of a metallic material with low electric resistance, for example, so that both the improvement in the bonding strength to bumps and improvement in the reduction of the ohmic loss are achieved.
Preferably, the first and second metallic films have a multi-layered structure formed by depositing a plurality of metallic layers, and the top metallic layer is preferably made of one of Ag and Au. In this case, since the top metallic layer in the first and second metallic films has excellent solderability, the first metallic film can be rigidly and readily bonded to an electrode land of the package with solder bumps therebetween.
Preferably, the interdigital electrode has a multi-layered structure formed by depositing a plurality of metallic layers, and wherein at least one metallic layer in the first and second metallic films is preferably made of a metal having a smaller specific electric resistance in comparison with the bottom metallic layer of the metallic layers constituting the interdigital electrode. Thereby, the specific electric resistance of at least one metallic layer in the first and second metallic films is relatively small, so that the ohmic loss is even further reduced in the metallic film while the thickness of the metallic layer made of a metal with relatively small specific electrical resistance is greatly reduced.
In accordance with a second preferred embodiment of the present invention, a manufacturing method of a surface acoustic wave element configured according to other preferred embodiments of the present invention includes the steps of forming the interdigital electrode, the bus bar electrodes, the relay electrodes, and the electrode pads on the piezoelectric substrate, forming the first metallic films on the electrode pads, and forming the second metallic films on at least one of the bus bar electrodes and the relay electrodes.
In accordance with a third preferred embodiment of the present invention, a communication apparatus includes a surface acoustic wave element configured according other preferred embodiments of the present invention as a band-pass filter.
In the surface acoustic wave element and the manufacturing method thereof according to preferred embodiments of the present invention, the second metallic film, preferably made of the same material as that of the first metallic film disposed on the electrode pad for increasing the bonding strength to the solder bump in the package side, is disposed on at least one of the bus bar electrode and relay electrode. That is, in the process for forming the first metallic film on the electrode pad for increasing the bonding strength to the solder bump in the package, also on at least one of the bus bar electrode and relay electrode, the second metallic film made of the same material is formed. Therefore, without complicating the manufacturing process, the electrical resistance loss of the electrode is greatly reduced. Thereby, an additional process is not required for increasing the film thickness of the bus bar electrode and relay electrode, so that the cost reduction for manufacturing the surface acoustic wave apparatus using the flip chip bonding technique can be achieved.
Additionally, since the increasing of the film-thickness of the bus bar electrode and relay electrode is performed by the metallic material having bonding strength to the conductive film constituting the IDT electrode, at least the same characteristics as those in the case of the film-thickness increasing with the same electrode material as those of the IDT electrode and relay electrode can be obtained.
Other features, elements, characteristics, steps and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.